1. Field of the Invention
The present invention relates to an engine control circuit that uses a processor to control the operation of an engine, more particularly to the prevention of a certain type of malfunction of the engine control circuit.
2. Description of the Related Art
Automobile engine control by means of a system including a processor such as a microcomputer is a known art: the system is generally referred to as an electronic control unit and more specifically as an engine control unit (ECU). ECUs having a power-down mode are also known: in the power-down mode, the ECU stops controlling the engine, and the ECU's power consumption is reduced to the minimum level necessary to retain the data stored in its random-access memory (RAM). The power source of an ECU is a regulator that draws current from the automobile's battery and converts the battery voltage to the voltage required by the ECU. Battery charge can be conserved by placing the ECU in the power-down mode while the engine is not running. A voltage detector in the ECU detects the voltage supplied to the igniter (the device that controls the engine's ignition) to decide whether the engine is running or not.
With such a voltage detector, however, there is the risk of a malfunction when the battery voltage drops rapidly. This is because, like the other circuits in the ECU, the voltage detector is supplied with power at a voltage converted from the battery voltage. Consequently, when the battery voltage drops rapidly, the power supply voltage of the voltage detector also drops, and this may cause the voltage detector to misjudge the battery voltage by deciding that the battery voltage is lower than it actually is. If the voltage detector detects a voltage lower than a predetermined threshold voltage, then even though the engine is still running, the ECU will decide that the engine has stopped and will enter the power-down mode. After entering the power-down mode, the ECU cannot control the igniter, so the engine actually does stop, unexpectedly.
The voltage of an automobile's battery drops, for example, when the engine is started. The cranking operation performed to start the engine places a maximum load on the starter motor, causing the output voltage of the battery to drop temporarily. The cranking operation takes place when the ignition switch is turned on by the automobile's ignition key, causing the starter motor to rotate and forcing the engine shaft to turn. A twelve-volt (12-V) battery, for example, has an open voltage of about thirteen volts (13 V), but this voltage drops to about nine volts (9 V) when the engine is started and then gradually increases to a stable value of about fourteen volts (14 V). A time of about one second elapses from when cranking starts until the battery voltage becomes stable.
The sudden temporary drop of the battery voltage when the engine is started becomes increasingly pronounced at low air temperatures. This is because a lowered air temperature increases the viscosity of the engine oil, thereby increasing the load on the engine shaft and accordingly the load on the starter motor.
Sporadic voltage drops, which are unlikely in a new battery, may also occur when the battery is degraded by age.
The regulator mentioned above can keep the ECU's power supply voltage steady as long as the battery voltage changes slowly. Since the regulator has a transient response time, however, it cannot follow rapid battery voltage changes, and may allow the ECU power supply voltage to drop for a period of, for example, about one millisecond.
One method of preventing temporary drops of the battery voltage is to use an auxiliary battery, for example, or a charged capacitor to compensate for the voltage drop, but this method does not fully prevent temporary voltage drops of the kind described above.
Other technology for preventing ECU malfunctions is disclosed in, for example, Japanese Patent Application Publication Nos. S62-258154 and H07-114401 and Japanese Patent No. H07-42888.
Japanese Patent Application Publication No. H07-114401 discloses an ECU that enters a power-down mode but retains its RAM data when the battery voltage drops below a predetermined voltage V1, and resets its central processing unit (CPU) when the battery voltage drops below a lower predetermined voltage V2. This first conventional scheme avoids such problems as loss of RAM data and CPU runaway when the battery voltage falls into the region between voltages V1 and V2, but does not prevent the battery voltage from being detected incorrectly when it drops suddenly. A sudden drop to a voltage slightly above V1 may still be detected as a drop to a voltage below V1, causing the ECU to power down and the engine to stop unexpectedly as described above.
Japanese Patent No. H07-42888 discloses an ECU that resets its CPU without resetting its RAM data when the battery voltage drops temporarily due to engine start-up. This second conventional scheme also avoids the problems of RAM data loss and CPU runaway when the battery voltage drops, but the ECU does not have a power-down mode. The ECU therefore cannot conserve battery charge by powering down when the engine is stopped, and the problems caused by mistaken entry into the power-down mode are not addressed.
Japanese Patent Application Publication No. S62-258154 discloses an ECU that, when the battery voltage drops temporarily, first inhibits the writing and reading of RAM data, then resets the CPU. Like the second scheme above, this third scheme avoids the problems of RAM data loss and CPU runaway when the battery voltage drops, but also as in the second scheme, the ECU does not have a power-down mode, so it cannot conserve battery charge by powering down when the engine is stopped, and the problems caused by mistaken entry into the power-down mode are not addressed.